Hardest for me is letting my Sims die or making them die for story purposes. My challenge Sim, Essa, was supposed to die of old age last year, but I just couldn't do it. So I made she and her husband vampires instead. The last time I let any of my Sims die was 4 years ago in the Apocalypse Challenge. For story purposes, I made my current Sim's wife die by Pufferfish Nigiri. I'm having a hard time dealing with it since the couple had just gotten married and have been together almost all this year. So, yeah...that's the toughest for me. :/